1. Field of the Invention
The present invention relates to detection devices, and more particularly to a remote motion detector for detecting the presence of an intruder within a secured area.
2. Description of the Prior Art
Heretofore most security detection devices have utilized signal reflections, such as radar, in order to determine the presence of intruders in a monitored area. In view of the cost and complexity of a typical radar system, various techniques have been developed in which linear devices like oscillators formed the detector base. Thus frequently but with some regularity certain intrusion detectors did utilize the inductive and capacitive effect of the entering body to provide detection. Typically, however, these last devices discriminate on the basis of frequency changes and the accuracy of the oscillator circuit is the dominant aspect. In order to improve this accuracy, or to reduce the unwanted registration of false alarms, various referencing techniques have been implemented wherein, for example, more than one oscillator would be driven, the detection being made on the basis of the differential taken therebetween. Alternatively a reference oscillator would be used in conjunction with the sensing oscillator to provide a base. In each instance the circuitry is necessarily doubled, increasing the cost and complexity of the system.
In addition, most of the prior art devices using either radar transmission or the capacitive and inductive effect, because of their frequency dependence, require highly accurate frequency control with the attendant sensitivity to component selection and the necessary isolation from external noise and internal drift.
Typically, any intrusion into a secured area entails the movement of a part or the whole of the body of the intruder with the attendant capacitive and inductive characteristics which are distinct from the ambient characteristics of the area monitored. These changes occur at cycle rates dictated by the scaling laws and therefore are quite limited in bandpass. In contrast, background noise is characteristically spread over a wide spectrum with the changes in the ambient conditions occurring at a very low rate and the electromagnetic noise appearing at rates substantially higher than those associated with body motion. Component drift and noise is similarly spread out. Thus, the component selection, temperature compensation and various other controls have to be effected in order to utilize systems as thus frequency dependent detection practiced in the prior art. Furthermore, the generation of a very high frequency imposes a very rigorous requirement as to the shape and consistency of the inductive component in the oscillator, once more resulting in frequency inaccuracies which demand constant attention.